1. Field of the Invention
The present invention relates to a method and an apparatus for structuring electrodesof an organic light-emitting device (OLED) used in a display unit. The present invention also relates to an organic light-emitting display manufactured using the method and apparatus.
2. Description of the Related Art
An organic light-emitting device is used in a display which represents symbols, images and the like. In an organic light-emitting device, an organic semiconductor layer (i.e. a light-emitting material layer) is positioned between two electrodes, for example, an anode and an electrode, and at least one of the electrodes is transparent for emitting light. The anode may be made, for example, of indium tin oxide which is transparent in the visible spectrum range. The indium tin oxide is deposited onto a glass substrate by a coating method. The cathode may be made, for example, of a metal, such as, for example, aluminum, which is vapor-deposited onto the organic semiconductor layer. When a voltage is applied to the electrodes, light is emitted and the color of emitted light is determined by the organic semiconductor layer. To emit light, positive charge carriers (also called “defect electrons” or “holes”) from the anode as well as electrons from the cathode are injected into the organic semiconductor layer. Once the electrons and the holes meet, neutral excited molecules are formed. While emitting light, the excited molecules are deactivated to the ground state.
To provide a high-resolution display, the cathode and the anode are patterned to form a matrix. In the matrix, intersections of the two electrodes form pixels or picture point.
Generally, a first electrode, which is typically an anode, is arranged on a substrate, such as, for example, a glass substrate, and an organic semiconductor layer is applied to the first electrode. Thus, the first electrode can be structured relatively easily with, for example, photolithographic technology. However, it is difficult to structure a second electrode, which is typically a cathode, because, during the structuring process, the material of the organic semiconductor layer is susceptible to chemical and/or thermal factors. The chemical and/or thermal factors may adversely affect the organic semiconductor layer.
Various methods for structuring the second electrode are known.
For example, deposition, and more specifically, vapor-deposition may be used to provide the material for forming a cathode on an organic semiconductor layer. A so-called shadow mask, as disclosed in U.S. Pat. Nos. 6,153,254 and 2,742,192, may be used to structure the material. However, during vapor deposition, the shadow mask is subjected to thermal stress and can be contaminated with, for example, the materials being deposited over a period of time. Thus, the shadow mask should be regularly cleaned and replaced. However, regularly cleaning and/or replacing the shadow mask may be complicated and work-intensive. In addition, when a larger shadow mask is used for a larger substrate, it is difficult for the central portion of the substrate to have a high resolution because gravity affects the shadow mask which is positioned above the substrate.
However, if an overhanging photo-resist layer is applied to the first electrodethe uppermost electrode may be structured without a shadow mask, as disclosed, for example, in European Patent EP 0 910 128 A2. The overhanging structure allows the second electrode to be structured while the material for the second electrode is being vapor-deposited. However, even a little damage or defect in the overhanging structure may, for example, prevent an individual line of two electrodes from being separated from each other.
Another method for structuring an upper electrode is, for example, to use photolithographic technology and/or lift-off technology. However, application of water-containing chemicals may cause damage to the semiconductor layer, for example, because the organic semiconductor layer may be exposed to water. To avoid such water damage, a more expensive and complex device is needed.
Laser ablation may also be employed to structure an upper electrode. In laser ablation, at least some of the portions of a homogeneous cathode layer which are unessential to a cathode structure are eliminated from the cathode by means of a laser beam. Use of laser ablation technology in the production of organic light-emitting devices is disclosed, for example, in EP 0 758 192 A2, WO 98/53510, WO 99/03157 and U.S. Pat. No. 6,146,715 as well as in Microfabrication of an Electroluminescent Polymer Light Emitting Diode Pixel Array by Noach et al. (hereinafter “Noach”), Applied Physics Letters, Vol. 69, No. 24, 1996, pages 3650–3652.
In the aforementioned and other known laser ablation technologies, a point-shaped laser profile is applied. That is, a laser beam scans across the cathode surface and, more particularly, the laser beam is applied to the desired electrode surface portion using a deflection mirror or any optically active device. However, galvanometer units and/or deflection units, for example, can be costly, and thus the cost of using laser ablation technologies can be high. In addition, such laser scanning may give rise to an overlap, and thereby result in a non-uniform display screen because when a small mirror in the galvanometer unit is subjected to specific inertia with a high deflection frequency, excess oscillation is generated. In the method disclosed in Noach, for example, a mesh strip, as used in an electronic microscope, is directly applied to an electrode surface to be ablated. In this case, the mesh represents the resolution of the laser. While an excimer laser scans across the mesh, scanned electrode materials are eliminated through the openings of the mesh. However, the mesh and the electrode surface are in direct contact with each other, and thus the mesh may be damaged.